openvswitch: correctly fragment packet with mpls headers

If mpls headers were pushed to a defragmented packet, the refragmentation no
longer works correctly after 48d2ab60 ("net: mpls: Fixups for GSO"). The
network header has to be shifted after the mpls headers for the
fragmentation and restored afterwards.

Fixes: 48d2ab60 ("net: mpls: Fixups for GSO")
Signed-off-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>